Editing Atmospheric entry (section)

===Refractory insulation===
[[File:Shuttle heat shield.jpg|thumb|left|Astronaut [[Andy Thomas|Andrew S. W. Thomas]] takes a close look at TPS tiles underneath [[Space Shuttle Atlantis|Space Shuttle ''Atlantis'']].]]
[[File:Silica Space Shuttle thermal protection (TPS) tile, c 1980. (9663807484).jpg|thumb|right|Rigid black [[LI-900]] tiles were used on the [[Space Shuttle]].]]

Refractory insulation keeps the heat in the outermost layer of the spacecraft surface, where it is conducted away by the air.<ref name=TPSpaper>{{cite conference
| url = https://ntrs.nasa.gov/citations/20160001151
| title = Thermal Protection Systems: Past, Present and Future
| last1 = Johnson
| first1 = Sylvia M.
| date = January 25, 2015
| publisher = 
| book-title = 
| pages = 
| location = International Conference and Exposition on Advanced Ceramics and Composites (Daytona Beach, FL)
| id = ARC-E-DAA-TN29151
| conference = 
| access-date = September 5, 2021
| archive-date = September 5, 2021
| archive-url = https://web.archive.org/web/20210905190358/https://ntrs.nasa.gov/citations/20160001151
| url-status = live
}}</ref> The temperature of the surface rises to incandescent levels, so the material must have a very high melting point, and the material must also exhibit very low thermal conductivity. Materials with these properties tend to be brittle, delicate, and difficult to fabricate in large sizes, so they are generally fabricated as relatively small tiles that are then attached to the structural skin of the spacecraft. There is a tradeoff between toughness and thermal conductivity: less conductive materials are generally more brittle. The space shuttle used multiple types of tiles. Tiles are also used on the [[Boeing X-37]], [[Dream Chaser]], and [[SpaceX Starship (spacecraft)|Starship's upper stage]].

Because insulation cannot be perfect, some heat energy is stored in the insulation and in the underlying material ("thermal soaking") and must be dissipated after the spacecraft exits the high-temperature flight regime. Some of this heat will re-radiate through the surface or will be carried off the surface by convection, but some will heat the spacecraft structure and interior, which may require active cooling after landing.<ref name=TPSpaper/>

Typical [[Space Shuttle thermal protection system|Space Shuttle TPS]] tiles ([[LI-900]]) have remarkable thermal protection properties. An LI-900 tile exposed to a temperature of 1,000 K on one side will remain merely warm to the touch on the other side. However, they are relatively brittle and break easily, and cannot survive in-flight rain.<ref>https://ntrs.nasa.gov/api/citations/19880011857/downloads/19880011857.pdf Damage testing on effect of Rain by Robert R. Meyer and Jack Barneburg</ref>